Assessing the l Extent of Drought George TSAKIRIS, Dialecti PANGALOU, Dimitris TIGKAS, Harris VANGELIS Lab. of Reclamation Works and Water Resources Management School of Rural and Surveying Engineering National Technical University of Athens 9 Iroon Polytechniou, 15780, Athens Greece e-mail: water@survey.ntua.gr Abstract: Key words: Drought is a three dimensional natural phenomenon characterised by its severity, duration and areal extent. The study deals with the areal extent of drought proposing two ways of its illustration and assessment: Discretised severity maps and or more cumulative curves of the affected area in relation to severity level. Using a reference period (e.g. year) instead of duration and a severity index such as the Standardized Precipitation Index () or the Reconnaissance Drought Index (), discretised maps show the areas affected by the corresponding severity of drought whereas the or more cumulative curves produce directly the percentages of areas belonging to each class of drought severity. Finally an illustrative application of both ways of drought areal extent estimation is presented for Eastern Crete, an area facing frequent droughts. areal extent, drought indices, or more curves, Reconnaissance Drought Index, Standardized Precipitation Index. 1. INTRODUCTION Drought is a recurring natural phenomenon characterised by its sevirity, duration and areal extent. It is therefore a three-dimensional phenomenon, which is difficult to assess. Significant work has been done over the last decades for devising indices, which represent the severity of drought (Yevjevich, 1967, Yevjevich et al., 1983, Rossi et al., 1992). Although the severity of drought could be rationally assessed from the anticipated consequences, drought indices have been widely used due to their simplicity, which they offer to both scientists and water management professionals. Some of the widely used drought indices are the PDSI (Palmer, 1965), the Deciles (Kinninmonth et al., 2000), the (McKee et al., 1993) and many others. Comprehensive reviews on these indices may be found in specific papers and publications (Richard and Heim, 2002, Hayes, 2004, Tsakiris et al, 2005, etc.). Recently a new index was proposed by Tsakiris (2004), the Reconnaissance Drought Index which uses potential evapotranspiration in conjunction with precipitation as the variables, which the drought assessment is based upon. In its Water Resources Management: New Approaches and Technologies, European Water Resources Association, Chania, Crete - Greece, 14-16 June 2007.
2 standardized form, in most of the cases, the responds in a similar way as the and the various thresholds representing the borders of severity classes are the same (Tsakiris and Vangelis, 2005, Tsakiris et al., 2006). By fixing the time reference of analysis, the duration dimension may be omitted. If for instance the hydrological year is selected as the reference period (that is from October to next September for the Mediterranean countries), drought can be assessed for each year of the historical record based on the selected drought index and its spatial distribution. 2. OVERVIEW OF DROUGHT INDICES For the purposes of this paper two drought indices were used: the and the. The Standardized Precipitation Index () was developed for the purpose of defining and monitoring drought based on a single meteorological determinant, the precipitation (McKee et al., 1993). The calculation for any location is based on a series of accumulated precipitation for a fixed time scale of interest (i.e. 1, 3, 6, 9, 12, months). Such a series is fitted to a probability distribution, which is then transformed into a normal distribution so that the mean for the location and desired period is zero (Edwards and McKee, 1997). Positive values indicate greater than median precipitation, and negative values indicate less than median precipitation. Because the is normalised, wetter and drier climates can be represented in the same way, and wet periods can also be monitored using the. The (Reconnaissance Drought Index) is based on the ratio between two aggregated quantities of precipitation and potential evapotranspiration. It appears in three forms: the initial value a k, the Normalised ( n ) and the Standardised ( st ). For real world applications if is calculated as a general index of meteorological drought it is advisable to use periods of 3, 6, 9 and 12 months. In its initial formulation for a 12 month period could be directly compared with the Aridity Index produced for the area under study. If a 12 for a certain year is lower than Aridity Index calculated according to UNEP (1992) then the area is suffering from drought during this year. The Standardised ( st ), behaves similarly to the and therefore the interpretation of the results is similar since the same thresholds as can be used. The equasions for calculating are presented in the Appendix. 3. DROUGHT SEVERITY MAPS AND CUMULATIVE OR MORE CURVES Drought severity maps have been used in many studies during the last decade (Kim et al., 2002, Tsakiris and Vangelis, 2004, Loukas and Vasiliades, 2004). Based on a descretisation procedure, the area under study can be divided into a number of
3 squares, at which the meteorological variables are transferred. Various methods for this transfer may be used (e.g. the weight of each station is taken as a reciprocal of square distance between the centre of the square in question and the location of the meteorological station in operation). Altitude correction should be also introduced if it is nessessary. Then, the selected drought index is calculated at each square for the reference period. Using different colours for each severity class a drought severity map is produced. Drought maps are very important tools for delineating the particular parts of the area under study, which are affected by drought. Although drought maps are useful, they exhibit two major drawbacks: a) They do not assess drought at the river basin level, at which water resources management is practised. b) They do not illustrate the areal extent of drought in relation to the threshold value of the critical area percentage. In order to correct the first drawback an additional drought map can be produced by a spatial integration, so that drought is expressed at the river basin scale. For this purpose it is advisable to transfer the values of the meteorological determinants to the river basin and particularly at the actual mean elevation of the basin. In case the basin is very large subbasins could be used as territorial units. As regards the second drawback, this work proposes to use the cumulative or more curves (ogives), also known as drought severity areal extent curves, which express directly the percentage of the area under drought, which then can be compared with the critical area percentage. It is customary to compare the areal extent of drought with a preset critical area percentage. These curves can be produced by plotting the severity of drought (y-axis) versus the percentage of the affected area (xaxis). The severity of drought is presented by a drought index and the area refers to that affected by at least the corresponding severity level. This type of graphs can be used not only for the characterisation of drought and the determination of its areal extent, but also for comparisons with the critical area percentage (related to severity) directly. Clearly, more than one thresholds referring to the percentage of critical area can be used defining different levels of severity. Since each class of drought severity has a different threshold, it is obvious that various critical area percentages could be simultaneously adopted for characterising a drought episode in relation to its areal extent. 4. APPLICATION For the application of the proposed methodology, the area of Eastern Crete - Greece (Prefectures of Heraklion and Lassithi) was selected. Both annual and were calculated, for the period of 30 hydrological years. Maps for one wet and one dry year (1964-65 and 1969-70 respectively) appear in Fig.1. As expected and give comparable results. However discripancies occur due to the fact that is using an additional meteorological determinant
4 (PET) apart from precipitation. Further in Fig.2 the most dry year (1989-90) during the examined period from 1962-63 to 1991-92 is presented. In Fig.3, the cumulative or more curves for both and are presented for the wet year 1964-65 and the dry year 1969-70. The boundaries of the drought severity classes [0,-1] normal to dry, [-1, -1.5] moderate dry, [-1.5, -2] severe dry and [<-2] extreme dry appear also in Fig 3. Focussing on the dry year 1969-70 it may be observed that various percentages of critical area may be adopted and may be directly compared with the areal extent of each drought level. If the is considered it can be seen that about 10% of the total area is under extreme drought, whereas 25% of the area is under extreme or severe drought and is is under at least moderate drought. In other words, 10% of the area is under extreme drought, 15% under severe drought and of the area is under moderate drought. Finally in Fig.4 the cumulative or more curves for the most dry year (1989-90) during the period examined are presented. From Fig.4 it can be concluded that more than of the area is under extreme drought. 1964 65 1969 70 Figure 1. Drought Severity Maps for Eastern Crete for one wet (1964-65) and one dry year (1969-70), based on the severity indices and
5 1989 90 Figure 2. The most dry year (1989-90) during the examined period 1962-63 1991-92 for Eastern Crete based on the and the. (1964-1965) (1964-1965) - - - - 10% 0% 10% 0% (1969-1970) (1969-1970) - - - - 10% 0% Figure 3. The cumulative or more curves for both and for the wet year 1964-65 and the dry year 1969-70. 10% 0%
6 (1989-1990) (1989-1990) - - - - 10% 0% 10% 0% Figure 4. The most dry year 1989-90 during the period 1962-63 1991-92 5. CONCLUDING REMARKS The proposed methodology showed that Drought Discretised Maps and Cumulative or more curves can be presented to illustrate the severity of drought and its areal extent. In particular, the cumulative or more curves describe the areal extent of drought in direct percentage figures, which may be compared with the adopted critical area percentage for each class of drought severity. Based on this information, a comprehensive analysis of past droughts can be assessed and sound preparedness plans can be devised for facing future droughts. ACKNOWLEDGEMENTS The paper is based on the research partially funded by the INTERREG IIIB - MEDOCC Programme of the European Commission within the framework of the SEDEMED II Project. REFERENCES Edwards D.C., McKee T.B.: 1997, Characteristics of 20th century drought in the United States at multiple time scales. Climatology Report Number 97 2, Colorado State University, Fort Collins, Colorado. Hayes M.: 2004, Drought Indices, National Drought Mitigation Centre. http://www.drought.unl.edu/whatis/indices.htm.
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